Problem: What is $90\%$ of $100$ ?
Answer: Having $90\%$ of something means that you get $90$ out of every $100$ We can set up a proportion to find out what number is $90\%$ of $100$ $ \dfrac{{\text{percent}}}{100} = \dfrac{{\text{part}}}{{\text{whole}}}$ Which things do we know, and what are we trying to find? We know the ${\text{percent}}$ is $90$ . Is $100$ the ${\text{part}}$ or the ${\text{whole}}$ The $100$ is the ${\text{whole}}$ . We are trying to find the ${\text{part}}$ that makes up $90\%$ of it: $ \dfrac{{90}}{100} = \dfrac{{\text{part}}}{{100}}$ If we multiply the denominator of the fraction on the left by $1$ , it will be the same denominator of the fraction on the right. To keep things equal, let's also multiply the numerator on the left by $1$ $ \dfrac{{90} \times 1}{100 \times 1} = \dfrac{{\text{part}}}{{100}}$ $ \dfrac{{90}}{100} = \dfrac{{\text{part}}}{{100}}$ $ {90} = {\text{part}}$ So $90$ is $90\%$ of $100$.